The present invention relates to a cathode ray tube having an electron gun with a bi-potential lens.
Various types of electron lenses are used when focusing electron beams in electron guns. A type frequently used is the bipotential electron lens consisting of two axially spaced apart, concentric cylinders. In operation a voltage difference is applied across the cylinders with the cylinder nearer the screen of the cathode ray tube being typically at the screen voltage. An example of such an electron lens is used in the electron gun system of the 30AX color display tube manufactured by N.V. Philips' Gloeilampenfabrieken.
When using an accelerating bipotential lens, one is bound to a given voltage difference between focusing voltage, that is the voltage applied to the first of the two cylinders in the electron beam path from the cathode to the screen, and the accelerating voltage, normally the screen voltage, applied to the second of the two cylinders. This voltage difference is determined by taking into account the distance from the lens to the object, the distance from the lens to the screen and the diameter of the lens. In the case of large diameter electron lenses, such as may be used in projection television tubes, the value of this voltage difference is such that a low focusing voltage, which is often unacceptably low, is the result. A theoretical study of electron lenses has led to the introduction of a quality factor (C), which includes the spherical aberration which is regarded as the dominant lens defect. The factor C raised to the power 1/4, that is C.sup.1/4, is directly proportional to the spot size produced at the screen. It was found that the bipotential lens yielded more favorable results at equal values of the lens-to-screen distance (Q), object-to-lens distance (P) and lens diameter (D) than the so-called uni-potential lens. The results could be summarized in the simple quality factor formula: ##EQU1## in which K has a value on the order 1.5 for a bipotential lens. The formula indicates that the quality increases as the diameter of the lens increases. However such large diameters cause problems because with a fixed screen voltage on the second cylinder, the focusing voltage on the first cylinder decreases with an increasing lens diameter in order to obtain the desired given voltage ratio. Such a low focusing voltage consequently occurs at the end of the triode part of the electron gun making it difficult to achieve the desired angle of aperture generated by the triode part. The presence of a high voltage in the triode part of the gun is required to achieve the desired angle of aperture. Moreover, the use of a high voltage in the triode region enables the introduction of a strong prefocusing lens. Proper positioning and strength of this prefocusing lens reduces the spot size. The solution of using a prefocusing electrode which is held at a voltage greater than that applied to the focusing electrode can be simplified if it is possible to electrically connect this electrode to the focus voltage or to mechanically incorporate this prefocusing electrode in the focus cylinder.